In the field of integrated circuit (IC) devices, there is often a need to detect a current at a particular point within a circuit, for example to enable control of other parts of the circuit, say via a feedback control loop. One way of implementing a suitable current sensor is to connect a resistor in series with the current path and measure the voltage across the resistor, which is proportional to the current flowing though the circuit. The drawback with this type of current sensor is that it is generally high loss, due to the ‘resistive’ nature of the resistor being incorporated into the current path. A potential solution could be to reduce the resistance value of the resistor; however this has the knock-on effect of reducing the sensitivity of the measuring device in the current path. Further, this type of resistive current sensor is only applicable to direct current (DC) circuits.
Another option for measuring currents within a circuit could be to utilise magnetic coupling, for example in the form of a Rogowski coil. Generally, in this type of circuit, a coil of metal is wound around, or in close proximity to, a current carrying wire, for example. If the coil and current carrying wire are in close proximity with each other, they will act as coupled inductors. Therefore, any change in current within the current carrying wire will be detected by the coil of wire, whereby the signal induced within the coil will be proportional to the change in current within the current carrying wire. In this way, the change in current can be ‘sensed’ due, in part, to mutual inductance. This method of current sensing has the advantage that the sensor (coil) is not directly coupled to the current carrying wire. Therefore, there is a degree of isolation between the sensor and the current carrying wire, thereby limiting any disturbance to the current flowing through the current carrying wire.
The method of current sensing discussed above generally has high parasitic capacitance and inductance, due, in part, to a plurality of turns being required to increase the sensitivity of the Rogowski coil or sensing coil. The increased sensitivity is achieved at the expense of lower bandwidth. It is well known in the art that parasitic capacitance and inductance collectively form the reactance of an AC circuit, the imaginary part of impedance. Thus, sensing coils, such as Rogowski coils, are not generally suitable for high frequency current sensing applications.